Energy consumption in the United States and throughout the world continues to increase. As the demand for energy increases, additional methods for producing energy are developed. Concerns about the increased waste and pollutants produced by many of the conventional energy production processes, and the low efficiencies of such processes, have led to further research for cleaner, safer, and more readily available energy sources. In particular, development and application of both renewable energy technologies and energy policy has led to increasing renewable energy utilization (solar, wind, geothermal) for power and energy production. Large-scale energy storage development is envisioned as a key requirement to enable significant penetration of renewable resources into the electric grid.
In response to increasing energy production requirements and the desire to reduce or eliminate pollutants from energy sources, new, cleaner fuel sources are being sought for a variety of applications, especially for transportation fuels. A known source of cleaner fuels includes synthetic fuels made from synthesis gas, or syngas, by so-called gas-to-liquid conversion processes.
However, standard methods for the production of syngas are fraught with technical challenges and, as a result, cost challenges. Syngas is produced by steam reforming of methane, coal gasification or biomass gasification. Steam reforming catalytically reacts methane gas with water to form syngas. The process operates at high temperature (greater than about 800° C.) and pressure (as high as about 100 bar) and typically requires expensive solid catalysts. Methane-steam reforming is one of the most common and cost-effective methods for the production of syngas, but it also consumes a gaseous fossil fuel in order to provide the feedstock to a liquid fuel synthesis reactor.
Coal gasification refers to the thermochemical conversion of coal, in a limited oxygen environment, to produce syngas. However, the syngas product stream resulting from coal gasification typically contains numerous contaminants, including benzene, toluene, volatile organics, polycyclic aromatic hydrocarbons, halides, mercury, hydrogen and carbonyl sulfides, and tars. Since many downstream gas-to-liquid conversion processes require a clean syngas feed stream, syngas from coal gasifiers requires additional treatment to remove these contaminants, including particulate scrubbing, halide removal, mercury removal, tar mitigation, and desulfurization. The cleaning steps result in higher capital and operating costs, which in turn increase the cost per standard cubic foot of clean syngas produced (unit cost).
Biomass gasification faces many of the same syngas contaminant challenges. Thermochemical conversion of biomass (e.g. cellulose, hemicellulose, lignin) produces a wide range of compounds, including oxygenates, aromatic hydrocarbons, phenolics, in addition to benzene, toluene and higher molecular weight tars. One study identified 230 separate chemical compounds formed by the thermal degradation of wood, in addition to carbon monoxide and hydrogen. As a result, syngas produced by biomass gasification, like coal gasification, requires significant processing before the syngas may be used in a gas-to-liquid process. Syngas tar removal is particularly complicated and costly, requiring processing steps like wet and/or dry scrubbing towers, tar cracking, acid gas removal, demisters, coalescers, and/or candle filters. This additional processing increases costs, energy requirements, and ultimately increases syngas unit cost.
The cost to produce syngas directly impacts the economic viability of the downstream gas-to-liquid conversion processes. One such process is the Fisher-Tropsch process, which converts syngas to long chained alkanes, including diesel and jet fuel. However, even today's Fisher-Tropsch processes are complex, costly and energy intensive. As a result, Fisher-Tropsch technology has seen limited industrial-scale use. On exception is South Africa, a country with large coal reserves but very little oil. The economic viability of Fisher-Tropsch technology in most of the world depends on the costs of crude oil compared to the costs of producing alternative fuels. However, given the historically volatile nature of natural gas costs, the political uncertainty of coal, and the rising global demand for crude oil, alternative syngas and gas-to-liquid technologies, and methods to integrate these technologies, is needed. In addition, linking the upstream syngas processes with downstream gas-to-liquid conversion processes requires innovative and novel heat integration methods. Otherwise, such new alternative energy concepts may not be able to compete with the relatively low cost of crude oil.
One promising new option for syngas/synfuel production is high temperature electrolysis technology which can also utilize intermittent renewable energy and provide energy storage in the form of liquid fuels. High temperature solid-oxide fuel cells may be used to produce electricity and water from hydrogen and oxygen. When run in reverse, a solid-oxide fuel cell acts as a solid-oxide electrolytic cell (SOEC), which is capable of electrolytically reducing water and carbon dioxide into hydrogen, carbon monoxide, and oxygen. Thus, water and carbon dioxide may be directly converted into a clean syngas (made up of hydrogen and carbon monoxide). In a solid-oxide fuel cell, the fuel electrode (i.e., the anode) is the oxidizing gas electrode and the air electrode (i.e., the cathode) is the reducing electrode. When operated in reverse, as a solid-oxide electrolytic cell, the polarity of the cell is switched in which the fuel electrode becomes the cathode reducing the incoming reactants and the oxidant electrode becomes the anode. It has been shown that an SOEC may be used for high temperature co-electrolysis of water and carbon dioxide to produce clean hydrogen and carbon monoxide, thus eliminating the costly tar and contaminant removal steps present in coal and biomass gasification processes.
The present invention provides a novel method for producing syngas, integrated with a gas-to-liquid conversion process that addresses the above-mentioned problems.